The Influence of Current Supply Type and Arc Length on C-Mn, C-Mn-Ni, and C-Mn-Ti-B Shielded Metal Arc Deposit Nitrogen and Oxygen Contents

Abstract

Nitrogen and oxygen contents have been determined for welds simulating root and hot passes, and deposited with progressively longer arc lengths to give arc voltages in the range 18 to 28 V. Commercial basic-coated C-Mn, C-Mn-Ni, and C-Mn-Ti-B shielded metal arc electrodes were employed in the flat (1G) position with DC straight polarity (electrode negative), DC reverse polarity (electrode positive), and alternating current (AC) supplies. In all deposits, nitrogen content increased with increasing arc length, with the C-Mn-Ti-B deposits showing the most rapid rise. The highest low voltage nitrogen levels and the most rapid increases always occurred for DC straight polarity welding, which gave >0.06% nitrogen at the highest arc voltage for each of the consumables. Deposit nitrogen content always exceeded the level in the core wire, but approached it at the lowest voltage for DC reverse polarity welds, with two of the C-Mn-Ti-B deposits showing the lowest nitrogen contents. Deposit oxygen contents were not related to core wire levels. For DC straight polarity deposits, oxygen increased progressively over the voltage range studied. For DC reverse polarity deposits, oxygen decreased progressively for AWS E7016 type electrodes, and showed an initial decrease and a subsequent increase for AWS E7018 type electrodes. The AC deposits showed either a small increase or decrease at low arc voltages and an increase at higher arc voltages. The findings reflect the incomplete shielding provided by the gas shield and slag layer at high arc voltages, particularly for DC straight polarity welding. They suggest that reactions involving the molten droplets emanating from the electrode tip are important in influencing deposit nitrogen content, and that those occurring in the slag layer at the surface of the weld pool are important in influencing deposit oxygen content.